The invention relates to a progressive spectacle lens as set out in the preamble of patent claim 1, the lens having low swaying effects.
Progressive spectacle lenses (also called varifocal lenses, multifocal lenses etc.) are usually understood to be spectacle lenses having a different (lower) power in the region through which a spectacles wearer views an object located at a great distancexe2x80x94hereunder referred to as a distance portionxe2x80x94than in the region (near portion) through which the spectacles wearer views a near object. Located between the distance portion and the near portion is the so-called progressive zone in which the power of the spectacle lens continuously increases from that of the distance portion to that of the near portion. The magnitude of the power increase is also designated as addition power.
As a rule, the distance portion is located in the upper part of the spectacle lens and designed for viewing xe2x80x9cto infinityxe2x80x9d, whilst the near portion is located in the lower region and is particularly designed for reading. In spectacles for special applicationxe2x80x94those for pilots or for monitor work stations are mentioned as examplesxe2x80x94the distance and near portions may also be arranged differently and/or designed for other distances. Furthermore, it is possible for a plurality of near portions and/or distance portions and suitable progressive zones to be present.
With progressive spectacle lenses having a constant refractive index it is necessary, in order that the power may increase between the distance portion and the near portion, that the curvature of one or both surfaces continuously change from the distance portion to the near portion.
The surfaces of spectacle lenses are usually characterized by the so-called principal radii of curvature R1 and R2 at every point on the surface. (Sometimes also the so-called principal curvatures K1=1/R1 and K2=1/R2 are given instead of the principal radii of curvature.) Together with the refractive index n of the glass material, the principal radii of curvature govern the parameters frequently used for an ophthalmologic characterization of a surface:
Surface power=0.5xc2x7(nxe2x88x921)xc2x7(1/R1+1/R2)
Surface astigmatism=(nxe2x80x941)xc2x7(1/R1xe2x88x921/R2).
Surface power is the parameter via which an increase of power from the distance portion to the near portion is achieved. Surface astigmatism (more clearly termed cylinder power) is a xe2x80x9ctroublesome propertyxe2x80x9d, because an astigmatismxe2x80x94inasmuch as an eye does not have an innate astigmatism to be correctedxe2x80x94which exceeds a value of about 0.5 dpt results in an indistinctly perceived image on the retina.
Although any change of the curvature of the surface which is needed to achieve a surface power increase without vision being xe2x80x9cdisturbedxe2x80x9d by surface astigmatism can be attained relatively simply along a (plane or winding) line, considerable xe2x80x9cintersectionsxe2x80x9d of surfaces will result alongside this line, leading to a large surface astigmatism which more or less impairs the lens in regions alongside the mentioned line.
On a surface having a surface power increasing from the distance portion to the near portion it is therefore not possible for reasons of surface theory to xe2x80x9cmaintainxe2x80x9d the regions alongside a line (that is free from astigmatism or exhibits a predetermined astigmatism) free from physiologically disturbing surface astigmatism (Minkwitz Law).
Because the curvature in the distance portion must not change, it is relatively simple to design the distance portion of the progressive surface in such manner that the distance portion has a very small surface astigmatism ( less than 0.5 dpt) or even a surface astigmatism value of xe2x80x9c0xe2x80x9d within a large region. On the other hand, the xe2x80x9cqualityxe2x80x9d of the shaping of the regions lateral to the transition region is of decisive importance for the acceptability of the spectacle lens to the spectacle wearer concerned.
The basic object in designing every progressive spectacle lens is therefore to shape the lateral regions within the transition zone, and also the lateral regions of the near portion if necessary, so that the spectacle lens will be as acceptable as possible to the spectacle wearerxe2x80x94without any unacceptable impairment of the distance portion.
In order to achieve this basic object, the designing of a progressive spectacle lens surface contributing to power change has in the past started out from a line lying or winding in a planexe2x80x94also designated as a principal meridian or a principal linexe2x80x94as a xe2x80x9cbackbone for the design of the surfacexe2x80x9d. This line runs centrally along the surface from top to bottom, and its course approximately follows the points of penetration of the rays of sight through the spectacle lens surface during a movement, in particular a lowering, of the glance. The principal curvatures at each point on this line were chosen in such manner that the desired increase of surface power (also designated as addition power) from the distance portion to the near portion is achieved. Starting out from this line, the lateral regions of the surface were then (more or less) suitably computed.
For the shaping of the lateral regions a number of solutions have become known. During the early days of the computation of progressive spectacle lenses a purely surface-theoretical optimization of exclusively the progressive surface was performed, in which the foremost concern was that of substantially reducing as far as possible the disturbing surface astigmatism, or xe2x80x9cdisplacingxe2x80x9d the surface astigmatism into the lateral lower regions of the spectacle lens.
Typical of this manner of proceeding are the progressive surfaces for spectacle lenses known from U.S. Pat. No. 2,878,721 or DE-AS 20 44 639, in whichxe2x80x94provided that the progressive surface is the front surfacexe2x80x94the curvatures of intersections of the progressive surface with plane surfaces which extend horizontally (horizontal meridians) or are perpendicular to the principal meridian (orthogonal meridians) are conic sections or curves of higher order, the curvatures of which increase in the distance portion and decrease in the near portion. The transition between the increase of curvature of the meridians in the distance portion and the decrease in the near portion occurs in the progressive zone.
This manner of proceeding results in the power difference and therewith also the surface intersections being reduced in the lateral regions. However, it is of disadvantage that because of the reduction of the power difference in the lateral regions, it becomes difficult to achieve an oriented view through the edge region andxe2x80x94depending on the designxe2x80x94very disturbing swaying sensations are experienced when looking through the lateral regions. Furthermore, the surface power along horizontal meridians changes relatively strongly.
For this reason it was proposed in DE-Patent 28 14 936 that the curvature in the distance portion or in the near portion be allowed to decrease or increase only in a strip on both sides of the principal meridian, and that the change of curvature be reversed outside this strip.
However, common to all of these approaches is a purely surface-related optimization of the progressive surface. This purely surface-related approach can be found also in more recent patent publications, such as DE-C-42 38 067 or DE-C-43 42 234.
In the last-mentioned publication an approach is described in which certain conditions for the gradient of the surface astigmatism and the surface power must be maintained. Irrespective of the circumstance that these gradients are not at all defined, for example when the surface is described by means of cubic splines, this purely surface-related optimization also disregards certain physiological requirements.
Approaches made to optimize the progressive surface in a wearing position are described in EP-A-677 177, U.S. Pat. No. 4,606,622 and DE 196 12 284.
For computing a progressive surface in the wearing position, a wearing situation is established. This relates either to a particular user for whom the individual parameters have been specially determined in the respective wearing situation and the progressive surface has been separately computed and fabricated, or to average values as described for example in DIN 58 208 Part 2.
However, even the known spectacle lenses which have been computed for the wearing position still have a number of disadvantages:
Thus, with the progressive surfaces known from U.S. Pat. No. 4,606,622, peaks are present along the course of lines of equal surface astigmatism for the lines having a surface power value of 5 dpt and 7 dpt.
Possibly the reason for this may be that a mixed use has been made of surface related and wearing position related approaches in the computation.
DE-A-196 12 284 deals with a decrease of the mean power close towards the periphery. This is an inadequate approach, because also in the distance portion the power should not increase excessively towards the periphery. Primarily, however, because of the stipulations which are one-sidedly related to the near-portion, swaying sensations are evoked, particularly with rotating movements of glance.
In EP-A-0 911 672 the course of the mean power along a circle having a diameter of 40 mm is given. In this, the mean power, i.e. a surface-related parameter, has a global minimum and a global maximum. Even with such surfaces comparatively strong swaying sensations arise.
The invention is based on the object of further developing a progressive spectacle lens as set out in the preamble of the patent claim 1 in such manner that it will not only have a large distance portion and a large near portion, but also that swaying sensations are avoided, particularly upon rotating movements of glance.
Achievements of this object by the invention are characterized in claims 1 and 5.
According to the present invention no surface values are taken into account, but only parameters which relate to the wearing position, namely the deviation from a given astigmatism (which is 0 dpt for an astigmatism-free eye, or has the magnitude and cylinder axis of cylinder prescription values), and the mean xe2x80x9cas wornxe2x80x9d power D as a measure of the xe2x80x9cpowerxe2x80x9d of the spectacle lens.
The mean xe2x80x9cas wornxe2x80x9d power D is the mean value of the reciprocals of the image side focal intercepts Sxe2x80x2 minus the object distance, i.e. the object side focal intercept S, as given by
D=0.5xc2x7(Sxe2x80x21+Sxe2x80x22)xe2x88x92S
Sxe2x80x2=reciprocal of the image side focal intercept
S=reciprocal of the object side focal intercept.
According to the invention it has been realized that the cause of the swaying movements perceived as being extremely disturbing by spectacle wearers is precisely the smooth variation of the mean power along a circle having a diameter of 40 mm, which is to be achieved in EP-A-911 672 and which results in only one global maximum and one global minimum.
For this reason the spectacle lens of the invention, which is the first achievement of the object, is designed in such manner that the astigmatic deviation, i.e. the difference between the prescribed and the actual astigmatism along circles having a center lying 4 mm below the centration point, and having a diameter of 10 to 40 mm, fulfils the following conditions:
two local minima occur which on a xe2x80x9cright-hand side spectacle lensxe2x80x9d are located at 95xc2x0xc2x110xc2x0 (according to TABO) and at 280xc2x0xc2x110xc2x0, and on a xe2x80x9cleft-hand side spectacle lensxe2x80x9d at 85xc2x0xc2x110xc2x0 and 260xc2x0xc2x110xc2x0, and two local maxima occur which on both a xe2x80x9cright-hand side spectacle lensxe2x80x9d and also a xe2x80x9cleft-hand-side spectacle lensxe2x80x9d are located at 215xc2x0xc2x110xc2x0 and 335xc2x0xc2x110xc2x0.
It is particularly preferred when the astigmatic deviation has exactly two local minima and maxima. Furthermore, it is of advantage when the values of both minima are less than 0.15 dpt.
In particular, the values of both maxima may differ by not more than 0.2 dpt and may each be smaller than 1.2xc2x7Add.
With an alternative or cumulative achievement of object provided by the invention, the mean xe2x80x9cas wornxe2x80x9d power along circles having a center lying 4 mm below the centration point and a diameter of 10 to 40 mm fulfils the following conditions:
for each diameter a global minimum occurs at 90xc2x0xc2x110xc2x0 (according to TABO) and a global maximum at 280xc2x0xc2x110xc2x0,
for diameters less than 16 mm, the global minima and maxima also are the only local minima and maxima,
for a diameter of 40 mm a local minimum and a local maximum occur on both the nasal and the temporal side.
Furthermore, it is preferred when for diameters between 14 and 26 mm saddle points are formed at 200xc2x0xc2x115xc2x0 and at 355xc2x0xc2x115xc2x0, wherein with increasing diameter these saddle points merge to become local minima or maxima, respectively. The local minima may be located at 215xc2x0xc2x115xc2x0 and 340xc2x0xc2x115xc2x0, and the local maxima at 200xc2x0xc2x115xc2x0 and 355xc2x0xc2x115xc2x0.